'Consensus' on climate change: what that does and doesn't mean

Discussions of climate change often pit two polarized sides talking past each other, split along partisan political lines.

Little wonder that it can seem hard to sift the answer to a fundamental question: Just how certain is the science, and how much do scientists agree?

The answer isn’t simple in all its details, because climate itself isn’t a simple thing. But the answer is also in many ways less complicated than the media rhetoric implies.

Listen to climate scientists themselves, and what emerges is that consensus about the basics of climate science is very strong, while there is much less certainty about specifics such as how fast temperatures or sea levels can be expected to rise.

When it comes to core concepts of climate change on a global scale, the scientific agreement is broad and clear-cut: Climate change is happening. The Earth is getting warmer. Human activity is largely responsible.

“Our climate system is far simpler than people think,” says James White, director of the Institute of Arctic and Alpine Research at the University of Colorado in Boulder. He likes to remind people that there are basic laws of physics at play, as immutable as the law of gravity. Global climate depends on how much energy we get from the sun, how much energy is reflected back to space, and the amount of greenhouse gases in the atmosphere.

“It’s not a belief issue,” says Professor White. Fossil fuels have been pumping more carbon dioxide into the atmosphere, and “We don’t know of any example in which greenhouse gases don’t absorb the Earth’s radiation and therefore warm the Earth up.”

That doesn’t mean the impacts of human activity are the only factor affecting climate.

Long before humans began burning fuels, Earth has seen ice ages and warm periods that scientists attribute to small shifts in the planet’s tilt and modest variations in its orbit around the sun, among other things.

Now, however, climate scientists say a warming trend is occurring even though natural factors don’t seem to predict it.

When people talk about the “97 percent” consensus among international scientists, this is the fundamental issue to which they’re referring: that human-caused climate change is already occurring.

Where the consensus starts to disappear is when it comes to more specific questions and predictions – how much and how quickly will temperature rise? How much and how quickly will sea level rise? What will the local climate effects be for specific regions? Have we passed a point of no return, or can the changes be halted? The foundation of science, after all, is skepticism, and climate models can become extraordinarily complicated very quickly.

Another issue is time frame, notes Richard Alley, a geologist at Pennsylvania State University who specializes in climate research. People tend to be interested in predictions a year or a decade out, but right now, any climate predictions for that time scale are clouded by what he calls “noise:” natural phenomena that affect climate and are impossible to know in advance. These include El Niño and La Niña, volcanic eruptions, and small changes in the sun’s brightness.

“Looking out some distance, the natural variability is still important,” says Professor Alley. “It’s in a few decades that our decisions [around how much carbon dioxide we’re adding to the atmosphere] become really important.”

For now, here’s where some of the consensus – and disagreement – lies on some fundamental issues.

Sea-level rise

Again, the basic scientific consensus here is pretty clear: Sea level is already measurably rising, the pace is increasing, and it will continue to rise. In the past century, Global Mean Sea Level has risen between 4 and 8 inches, and in the past couple decades, the rate of increase has roughly doubled.

Scientists say this is one of the easiest effects of global warming to predict: As water heats up (as is happening) it expands, and as glaciers recede and ice sheets melt, their runoff contributes to the rise.

The biggest impact, however, is expected to come from melting land ice in Greenland and Antarctica, if that occurs in large amounts. The uncertainty around that land ice – along with uncertainty on the pace of change – is what leads to very different predictions.

The National Climate Assessment, which summarizes risks to the US, projects that sea level will rise between 1 and 4 feet (0.3 to 1.2 m) by 2100. According to the most recent report by the International Panel on Climate Change (IPCC), sea levels will rise between .26 meters and .82 meters by 2100 (roughly a 2-foot rise), depending on how emissions continue. That’s an estimate that most scientists see as very conservative, particularly given the amplification that can occur, if a triggering event happens to make a large chunk of an ice sheet break off.

“You can imagine our climate system is like a pot of water on the stove, and we’re turning up the heat, but the water’s not heated up yet,” says White.

Ultimately, he, along with many scientists, agree that the current levels of carbon dioxide in the atmosphere will lead to sea level rises between 10 and 20 meters – but it may take many centuries for that to happen. “We disagree on how high sea level will go at 400 ppm, and we disagree on where the thresholds are that would give you enough carbon dioxide to melt all of Antarctica,” explains White.

Natural disasters and regional change

One of the most controversial areas of climate change involves natural disasters. Will the world see more hurricanes and storms? More droughts and floods? Are we already seeing these impacts, and is climate change to blame for recent disasters?

The research is murky. The IPCC says that “it is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions.”

But there’s less data to back up increased frequency of hurricanes or tornadoes, and it can become very difficult to tease out, for any one natural disaster, whether a portion of its intensity is attributable to climate change.

And depending who you talk to, scientists offer very different predictions for regional change. “In a warmer planet, the atmosphere holds more moisture, and the potential for big rainfall gets larger and larger, so both floods and droughts increase,” says White. But which areas will get drier? Which might have more floods? “This is where the models struggle. Getting the hydrologic cycles right in a model is not easy,” he says.

How quickly will the planet warm?

Scientists have created varied complex models to make climate forecasts, and there is no consensus about how much, or how rapidly, the planet will warm. The most conservative models, assuming the lowest emissions, give an average temperature increase of about 2 degrees F over this century, while higher-emissions models suggest an increase of more than 11 degrees F over the century.

Even within a single baseline of assumptions about emissions, temperature forecasts can vary based on things like how global cloud cover will change under a given model.

Warming is likely to be more extreme at higher latitudes, and most models predict that summer sea ice will be gone from the Arctic by the end of the century.

Part of the reason for the disparity lies in the uncertainty about how carbon emissions may change over the coming century, and part lies in the complex ways in which small changes can have an amplifying effect, or a certain threshold can be crossed that leads to rapid change in a short period of time.

“We are raising CO2, [and] CO2 is having a warming effect on the climate: That’s as close to fact as scientists ever get,” says Alley. “There’s such an interwoven fabric of evidence at this point that any argument about it is noise.” And asking scientists about those basic facts gets 97 percent or more to agree pretty consistently, he says. “But if you write anything specific [around detailed predictions] you’ll find somebody will object to it, because there are several studies and they’re all slightly different.”

The debate among climate scientists isn't always just about the details of specific forecasts. A few scientists worry aloud that a kind of groupthink and peer pressure is compromising public confidence in the quality of published research, as academics see their careers as tied to toeing acceptable lines.

But that critique isn't widely shared. In fact, given all the inquiry into different climate issues, many scientists say the big need is actually to help the public get a better sense of the strong consensus that exists on the core points that global warming is real and that it threatens significant harm to ecosystems and human societies.